Cycloaddition of Carbodiimides with a Heteroaromatic Substituent to Allenic Acids

Cycloaddition of the allenic acid 3 with the N -cyclohexyl-N′ -heteroaromatic carbodiimides 2a and 2b gave the isomeric pyrido[1,2-a]pyrimidinones 4 and 5 and thiazolo[3,2-a]pyrimidinones 6 and 7 , respectively, instead of the expected Diels-Alder adducts analogous to 1 . The compounds of the latter type, i.e. 8 and 9 , were formed from 3 and carbodiimides 2c and 2d , respectively, containing an N′-(pyrazin-2-yl) or N′-(pyrimidin-2-yl) substituent.     

Comparison of bacterial and phage display peptide libraries in search of target-binding motif

Genetic engineering allows modification of bacterial and bacteriophage genes, which code for surface proteins, enabling display of random peptides on the surface of these microbial vectors. Biologic peptide libraries thus formed are used for high-throughput screening of clones bearing peptides with high affinity for target proteins. There are reports of many successful affinity selections performed with phage display libraries and substantially fewer cases describing the use of bacterial display systems. In theory, bacterial display has some advantages over phage display, but the two systems have never been experimentally compared. We tested both techniques in selecting streptavidin-binding peptides from two commercially available libraries. Under similar conditions, selection of phage-displayed peptides to model protein streptavidin proved convincingly better.     

The vibrations of formaldehyde

A new six-dimensional variational code is presented for the determination of the vibrational energy levels of four-atom molecules which are either linearly connected or centrally connected. Internal (displacement) coordinates are used. It is demonstrated for the CCSD(T) surface of formaldehyde, due to Martin, Lee and Taylor, and it is shown that a small scaling of the force constants leads to a high-accuracy surface.     

On n‐dependent groups and fields

First, an example of a 2‐dependent group without a minimal subgroup of bounded index is given. Second, all infinite n‐dependent fields are shown to be Artin‐Schreier closed. Furthermore, the theory of any non separably closed PAC field has the IP_n property for all natural numbers n and certain properties of dependent (NIP) valued fields extend to the n‐dependent context.     

Kinetic properties of hexameric tyrosinase from the crustacean Palinurus elephas

Tyrosinases catalyze hydroxylation of monophenols to o-diphenols and their subsequent oxidation to o-quinones, whereas catecholoxidases catalyze only the latter reaction. Both enzymes occur in all organisms and are Type 3 copper proteins that perform the first steps of melanin formation. In arthropods, they play an essential role in the sclerotization of the exoskeleton. Very few phenoloxidases are characterized structurally or kinetically and the existence of an actual tyrosinase activity has not been demonstrated in most cases. Here we present for the first time a complete kinetic characterization of a tyrosinase from a crustacean (Palinurus elephas) including the influence of inhibitors. In contrast to most tyrosinases which are monomeric or dimeric, this tyrosinase occurs as a hexamer. However, the data did not indicate cooperativity in steady-state kinetics for the two substrates used, the monophenol tyramine and the diphenol dopamine. Mimosine as well as phenylthiourea (PTU) inhibited both monophenolhydroxylase and diphenoloxidase activity. Inhibition by mimosine was competitive, whereas PTU was a noncompetitive inhibitor. Furthermore, for the diphenolase activity substrate inhibition was observed, which was apparently abolished by adding PTU. These observations lead to the hypothesis that a secondary, allosteric binding site exists, which binds dopamine and PTU and reduces the catalytic activity.     

Stability, dynamics, and lubrication of MoS2 platelets and nanotubes

A model is introduced to investigate structure, stability, dynamics, and properties of MoS(2). The tribological behavior of the material is obtained from the autocorrelation function, ACF, of the forces, using the Green-Kubo equation, and by the classical Amontons' laws. In the idealized system, i.e. without defects, junctions, vacancies, asperities, and impurities, both models find a superlubrication regime, in agreement with some experiments. In nanotubes, NTs, friction is an order of magnitude lower than in the layered systems. The calculations also show that there is a substantial stabilization, per atom, for the formation of multiwall NTs with at least four walls.     

On Bounded-Degree Vertex Deletion parameterized by treewidth

Given an undirected graph $G$ and an integer $d\ge 0$, the NP-hard Bounded-Degree Vertex Deletion problem asks to delete as few vertices as possible from $G$ such that the resulting graph has maximum vertex degree $d$. Our main result is to prove that Bounded-Degree Vertex Deletion is W[1]-hard with respect to the parameter treewidth. As a side result, we obtain that the NP-hard Vector Dominating Set problem is W[1]-hard with respect to the parameter treewidth. On the positive side, we show that Bounded-Degree Vertex Deletion becomes fixed-parameter tractable when parameterized by the combined parameter treewidth and number of vertices to delete, and when parametrized by the feedback edge set number.     

Barium isotope fractionation during experimental formation of the double carbonate BaMn[CO3](2) at ambient temperature

In this study, we present the first experimental results for stable barium (Ba) isotope ((137)Ba/(134)Ba) fractionation during low-temperature formation of the anhydrous double carbonate BaMn[CO(3)](2). This investigation is part of an ongoing work on Ba fractionation in the natural barium cycle. Precipitation at a temperature of 21±1°C leads to an enrichment of the lighter Ba isotope described by an enrichment factor of-0.11±0.06‰ in the double carbonate than in an aqueous barium-manganese(II) chloride/sodium bicarbonate solution, which is within the range of previous reports for synthetic pure BaCO (3) (witherite) formation.